The present invention relates to a process for the preparation of alkanolamines having improved color quality by treating the alkanolamine with hydrogen in the presence of a hydrogenation catalyst at elevated temperature.
Important fields of use of alkanolamines, such as, for example, triethanolamine (TEA), are, for example, soaps, detergents and shampoos in the cosmetics industry, and also dispersants and emulsifiers.
For these and other fields of use, water-clear, colorless alkanolamines having as little discoloration as possible, e.g. measured as APHA color number, which retain these properties even over prolonged storage time (of, for example, 6, 12 or more months) are desired.
It is known that a pure alkanolamine obtained by fractional distillation of an alkanolamine crude product which has, for example, been obtained by reacting ammonia with ethylene oxide or propylene oxide, and initially colorless (color number: about 0 to 20 APHA according to DIN-ISO 6271(=Hazen)), can, after a storage time of from about 4 to 6 weeks, even in a sealed container and with the exclusion of light, gradually turn pale pink or pale yellow and ultimately, particularly if left to stand in light, can turn yellow to brown. This effect is accelerated by the action of higher temperatures. (See e.g.: G. G. Smirnova et al., J. of Applied Chemistry of the USSR 61, p. 1508-9 (1988), and Chemical & Engineering News, Sep. 16, 1996, page 42, middle column.)
The literature describes various processes for the preparation of alkanolamines having improved color quality.
EP-A-4015 describes how mono-, di- and triethanolamine having a lower degree of discoloration are obtained by the addition of phosphorous or hypophosphorous acid or compounds thereof during or following the reaction of ethylene oxide with ammonia and prior to their isolation by distillation.
EP-A-36 152 and EP-A-4015 explain the influence of the materials used in processes for the preparation of alkanolamines on the color quality of the process products and recommend low-nickel or nickel-free steels.
U.S. Pat. No. 3,207,790 describes a process for improving the color quality of alkanolamines by adding a borohydride of an alkali metal to the alkanolamine.
U.S. Pat. No. 3,742,059 and DE-A-22 25 015 describe the improvement in the color quality of alkanolamines by the addition of an alkanolamine ester of boric acid or alkali metal/alkaline earth metal borates.
However, the presence of an auxiliary (stabilizer) for improving the color quality of alkanolamines is undesired in many important application areas.
The earlier German Application No. 19855383.8 dated Jan. 12, 1998 relates to a process for the purification of TEA prepared by reacting aqueous ammonia with ethylene oxide in liquid phase under pressure and at elevated temperature, by separating excess ammonia, water and monoethanolamine from the reaction product, reacting the resulting crude product with ethylene oxide at temperatures from 110 to 180.degree. C., and subsequently rectifying the mixture in the presence of phosphorous or hypophosphorous acid or compounds thereof.
U.S. Pat. No. 3,819,710 discloses a process for improving the color quality of ethanolamines by hydrogenating the crude ethanolamines in the presence of selected catalysts, such as, for example, Pt, Pd, Ru or, preferably, Raney nickel. The process does not lead to ethanolamine products which remain colorless over several months.
According to the invention, it has also been recognized that a general disadvantage when using Raney catalysts is that the reaction product contains undesired traces of aluminum since the alkanolamines act as complexing agents toward aluminum. This leads to permanent damage to the Raney structure and thus to a reduction in the stability and activity of these catalysts.
In addition, according to the invention it has been recognized that if Raney nickel or Raney cobalt are used as catalyst in the purification of alkanolamines, traces of Ni or Co are found in the reaction product since the alkanolamines also act as complexing agents toward nickel and cobalt.
EP-A-28 555 teaches a process for the purification of N,N-dialkylaminoethanols by a catalytic hydrogenation in the heterogeneous phase and subsequent rectification (cf. claim 1 and page 2, lines 23 to 30), where the catalyst comprises a metal chosen from group VIII of the Periodic Table of the Elements, such as, for example, Ni, Co, Pt, Rh or Pd.
JP-A-011 609 47 (Derwent Abstr. No. 89-224471/31, Chem. Abstr. 111:232081r (1989)) describes the purification of dialkylaminoethanol by the steps (a) removal of high-boiling impurities, (b) treatment with hydrogen in the presence of a hydrogenation catalyst which preferably comprises from 0.3 to 7% by weight of a metal from group VIII on a support (such as, for example, Ru/C), and (c) distillation.
According to the invention it has been recognized that many catalyst support materials, such as, for example, gamma-aluminum oxide and magnesium oxide, have the disadvantage that alkanolamines act as complexing agents toward the support material or individual components of the support material, the support material thus leaches out and, as a result, undesired support constituents are found in the reaction product.